Endotoxin is one of the primary mediators of inflammation released by Gram negative organisms and is an important cause of environmentally induced airway disease, such as ARDS. Inhaled endotoxin can cause airflow obstruction in previously unexposed subjects. Inhaled endotoxin is strongly associated with the development of acute decrements in airflow among cotton workers, wine confinement workers, and poultry workers. The concentration of endotoxin in the bioaerosol appears to be the most important occupational exposure associated with the development and progression of airway disease in agricultural workers (Schwartz, D. A., et al., Am. J. Respir. Crit. Care Med. 152:603-8, 1995).
In addition to being related to several occupational pulmonary diseases, exposure to endotoxin and to its purified derivative lipopolysaccharide (LPS) is also associated with severe asthma. The concentration of endotoxin in the domestic environment adversely affects asthmatics, with higher concentrations of ambient endotoxin associated with greater degrees of airflow obstruction. In addition, asthmnatic individuals develop airflow obstruction at lower concentrations of inhaled endotoxin than normal controls. Exposure-response studies have shown that inhaled lipopolysaccharide (LPS) produces recruitment of neutrophils, activation of macrophages with production and release of pro-inflammatory cytokines, and damage to airway epithelia in a dose-dependent manner. These studies indicate that endotoxin is an important cause of airway disease among exposed individuals.
The acute respiratory distress syndrome (ARDS) is a condition characterized by acute hypoxemia respiratory failure due to pulmonary edema (reviewed in Honing, E. G., and Ingram, R. H., Jr., in: Harrison's Principles of Internal Medicine, 14th Edition, A. S. Fauci, et al. (eds.), McGraw-Hill, N.Y., pp. 1483-1486, 1998; and Goodman, R. B., et al., Am J. Respir. Crit. Care Med. 154:602-11, 1996). ARDS represents a spectrum of responses to acute lung injury (ALI); these response occur as complications of a more widespread systemic response to acute inflammation or injury. ALI develops rapidly after a predisposing condition triggers a systemic inflammatory response and is most strongly associated with conditions that produced direct alveolar injury or direct injury via the pulmonary capillary bed, such as aspiration, diffuse infection, toxic inhalation, direct injury to the alveolar epithelium, or sepsis syndrome. ALI is the consequence of unregulated over-expression of usual systemic inflammatory responses to infection and/or injury. Injury involves the alveolar epithelium and the pulmonary capillary endothelium, and results in a complex cascade of events. Injury is produced by cellular events associated with neutrophils, macrophages, monocytes, and lymphocytes producing various cytokines, in turn producing cellular activation, chemotaxis, and adhesion.
Gram-negative infections are a major cause of morbidity and mortality, especially in hospitalized and immunocompromised patients. (Duma, Am. J. of Med., 78 (Suppl. 6A): 154-164, 1985; and Kreger et al., Am. J. Med., 68:344-355, 1980). Although available antibiotics are generally effective in inhibiting growth of Gram-negative bacteria, they do not neutralize the pathophysiological effects associated with endotoxins. Endotoxin is a heat stable bacterial toxin composed of lipopolysaccharides (LPS) released from the outer membrane of Gram-negative bacteria upon lysis (Shenep et al., J. Infect. Dis., 150(3):380-388, 1984), and is a potent stimulator of the inflammatory response. Endotoxemia occurs when endotoxin enters the bloodstream resulting in a dramatic systemic inflammatory response.
The uptake of oligonucleotides by B lymphocytes has been shown to be regulated by LPS-induced cell activation (Krieg, A. M., et al., Antisense Res. Devel. 1:161, 1991). Many detrimental in vivo effects of LPS have been shown to result from soluble mediators released by inflammatory cells. (Morrison et al., Am. J. Pathol., 93(2):527-617, 1978). Monocytes and neutrophils, which ingest and kill microorganisms, play a key role in this process. Monocytes and neutrophils respond to endotoxin in vivo by releasing soluble proteins with microbicidal, proteolytic, opsonic, pyrogenic, complement-activating and tissue-damaging effects. These factors mediate many of the pathophysiological effects of endotoxin. For example, tumor necrosis factor (TNF), a cytokine released by endotoxin-stimulated monocytes, causes fever, shock, and alterations in glucose metabolism and is a potent stimulator of neutrophils. Other cytokines such as IL-1, IL-6, and IL-8 also mediate many of the pathophysiologic effects of LPS, as well as other pathways involving endothelial cell activation by tissue factor, kininogen, nitric oxide and complement.
Endotoxin-associated disorders result from extra-gastrointestinal exposure to LPS, e.g., administration of LPS-contaminated fluids, inhalation of LPS, or Gram-negative infections. Endotoxin-associated disorders can also result when the natural cellular barrier is injured and the normal Gram-negative flora breach this barrier. For example, endotoxin-associated disorders can occur (a) when there is ischemia of the gastrointestinal tract (e.g, following hemorrhagic shock or during certain surgical procedures), or (b) when systemic or local inflammation causes increased permeability of the gut or lung to endotoxin or Gram-negative organisms. The presence of endotoxin and the resulting inflammatory response may result, for example, in adult respiratory distress syndrome (ARDS), dust-induced airway disease, and exacerbation of asthma, in addition to endotoxemia, systemic inflammatory response syndrome (SIRS), sepsis syndrome, septic shock, disseminated intravascular coagulation (DIC), cardiac dysfunction, organ failure, liver failure (hepatobiliary dysfunction), brain failure (CNS dysfunction), renal failure, multi-organ failure and shock.
Several therapeutic compounds have been developed to inhibit the toxic effects of endotoxin, including antibacterial LPS-binding agents and anti-LPS antibodies, although each has met with limitations. For example, Polymyxin B (PMB) is a basic polypeptide antibiotic which binds to Lipid A, the most toxic and biologically active component of endotoxin. PMB inhibits endotoxin-mediated activation of neutrophil granule release in vitro and is a potential therapeutic agent for Gram-negative infections. However, because of its systemic toxicity, this antibiotic has limited therapeutic use, and is generally used topically. Combination therapy using antibiotics and high doses of methylprednisolone sodium succinate (MPSS) showed more promise as this regimen prevented death in an experimental animal model of Gram-negative sepsis. However, a clinical study using MPSS with antibiotics in treatment of patients having clinical signs of systemic sepsis showed that mortality rates were not significantly different between the treatment and placebo groups (Bone et al., N. Engl. J. Med. 317:653, 1987).